Polymerization in aqueous emulsions

The purification procedures to be applied depend on the monomer, on the expected impurities, and especially on the purpose for which the monomer is to be employed, e.g., whether it is to be used for radical polymerization in aqueous emulsion or for ionic polymerization initiated with sodium naphthalene. It is not possible to devise a general purification scheme instead the most suitable method must be chosen in each case from those given below. A prerequisite for successful purification is extreme cleanliness of all apparatus (if necessary, treating with hot nitrating acid and repeatedly thorough washing with distilled water). 

Vinyl acetate is polymerized in aqueous emulsion and used widely in surface coating and in adhesives. Copolymerized with vinyl esters of branched carboxylic acids and small quantities of acrylic acid, it gives paint latices of excellent performance characteristics. G. C. Vegter found that a coagulum-free latex of very low residual monomer content can be produced from a mixture of an anionic and a nonionic emulsifier according to a specific operating procedure. The freeze/thaw stability of polymeric latices has been investigated by H. Naidus and R. Hanzes. 

Styrene, methyl methacrylate, ethyl acrylate, and methyl acrylate are easily polymerized in aqueous emulsion under the action of Co y-radiation. The maximum over-all reaction rate increases in the sequence given above. Using a dose rate of 200 rad per hour, the doses required to obtain a conversion of 80 to 90% amount to about 10 rad. This dose is far lower than the doses required for a measurable degradation or cross linking of the polymers formed. The G value of formation of polymer molecules was found to be orders of magnitude higher than the Gr (H2O) values referred to in the literature. 

By comparison, free-radical butadiene polymerization in aqueous emulsion typically yields polymers with 60% trans-incorporated units under similar conditions [22]. Polymer molecular weights of 2.6 x 10" g mol were determined by light scattering [21]. 

In 1993, Perez et al. reported norbornene polymerization in aqueous emulsion at 70 °C using PdCl2 as a catalyst precursor [92, 93]. A stable latex consisting of low-molecular-weight oligomeric material (degree of polymerization DP ca. 10) was obtained with low catalyst activities (70 TO h ). Very small latex particles of 10 to 20 nm diameter were reported. In the free radical polymerization of olefinic monomers such small particles are only obtained by microemulsion polymeriza-... 

Polymerization in aqueous emulsions is common industrial practice. The emulsion has a much lower viscosity than a comparable solution in a solvent. It is possible to obtain higher molecular weights in the emulsion because there is no solvent present to act as a chain-transfer agent. Frequently, the product emulsions can be used as such (e.g., as adhesives or coatings). 

Polymerization in aqueous emulsions, which has been widely developed technologically, represents a special case of free radical chain polymerization in a heterogeneous system [52-58]. Most emulsion polymerization systems... 

Sometimes, the two processes (reversible termination and reversible transfer) can operate simultaneously (example hving radical poljmerizations mediated by organo-cobalt porphjrin complexes reported by Wayland et al. ). Several CRP techniques have been developed in the past thirty years, but their implementation at an industrial scale, especially in heterogeneous aqueous processes which are of major importance nowadays, remains a challenge. In this paper, we report our strategy to set up a new CRP technique based on simple, readily available and economical chemicals and our attempts to control the polymerization in aqueous emulsion polymerizations. Experimental details are given elsewhere.l ... 

The basic monomer unit is a totally fluorinated ethylene molecule (—CF —CF —). It is well known under its common trade name Teflon. It was discovered in 1938 by Roy J. Plunkett a DuPont scientists. Industrially, polytetrafluoroethylene is obtained from several consecutive of steps. First, chloroform reacts with hydrofluoric acid to yield chlorodifluoromethane. The chlorodifluoromethane is then pyrolized at 800-1000 C to yield the monomer, i.e., tetra-fluoroethylene (CF2=CF2, TFE) which is purified and polymerized in aqueous emulsion or suspension using organic peroxides, persulfates or hydrogen peroxide as catalysts. The simple polymerization reaction is as follows. 

Norbornene can also be polymerized in aqueous emulsion by palladium chloride to give fine microlatex polymers unavailable by polymerization in anhydrous solvents [193]. 

Terpolymers ofTFE, HFP and VDF are melt-processable fiuoropolymers. They are polymerized in aqueous emulsion. Depending on the ratio of the monomers, a large number of different terpolymers are available. Figure 3.15 depicts the elastomeric region in the ternary system [28]. These materials are commercially available as TfJV (Dyneon) [32]. 

---